Conventionally, a technology that calculates a sight direction by detecting a region of an eye and/or a location of a center of a pupil from a face image captured by a camera is known. For example, in an invention in the patent document 1 listed below, a center of a pupil is detected by using the following process. First, a region of an eye on an image is detected by a template matching method. An edge or edges are detected within the region of the eye detected. Then, the circle Hough transformation is applied to the detected edge(s) in order to obtain a circle equation and to detect a center of the pupil. Then, the apparatus calculates a sight direction based on a relative relationship between the region of the eye and the center of the pupil.
Patent document 1: JP-2002-282210-A
However, an outline of a pupil cannot be always detected in a perfect form. A part of a pupil may hide below an eyelid depending on direction of a face or direction of a sight direction. In addition, since an eye shape also has a personal difference. Therefore, in some cases, a part of a pupil may hide below an eyelid even in a condition where a person faces front side. For example, there may be a case where an edge is detected when an upper part of a pupil is hidden below an upper eyelid as shown in FIG. 17A. Here, a pupil is a black region located on a center of an iris. In the drawings the pupil is illustrated slightly large. As shown in FIG. 17B, an edge of the upper eyelid and an edge of the pupil are seemingly detected in a continuous form. If the circle Hough transformation is performed to such edge, a result is influenced by the edge of the eyelid as shown in FIG. 17C. For this reason, detecting process may detect a location of a pupil with error component.